New research

Leaf-eating insects may limit how much carbon forests absorb, study says

  • 03 Mar 2015, 17:20
  • Robert McSweeney

Insects could restrict how well trees absorb and store carbon in the future, according to a new study. In experiments simulating carbon dioxide levels in 2050, insects munched their way through almost double the number of leaves than under current conditions.

With fewer leaves, the trees are likely to become less effective at absorbing carbon dioxide from the air, the researchers say.

Carbon dioxide fertilisation

Trees are the biggest carbon sink on land. Through a process known as photosynthesis, trees convert carbon dioxide, water and sunlight into the fuel they need to grow, locking up carbon in their trunks and branches as living biomass.

Research suggests that when there's more carbon dioxide in the air, trees grow more quickly because the rate of photosynthesis speeds up. This is called 'carbon dioxide fertilisation'.

Scientists expect that as human-caused carbon dioxide emissions increase, forests will absorb and store more carbon, assuming they have enough water and nutrients to grow.

But the results of a three-year experiment, just published in Nature Plants, suggests that insects, such as caterpillars, may make it harder for trees to absorb this extra carbon by munching their way through the trees' leaves. With fewer leaves to absorb sunlight, the trees can't photosynthesise as much, and they absorb less carbon dioxide from the air.

'Double whammy'

The researchers set up their experiment at the Aspen FACE facility (Free-Air Carbon Dioxide Enrichment) in Wisconsin in the northern US. At the facility, birch and aspen trees are surrounded by vertical vents that release different gases to simulate future conditions.

FACE Aerial 2005 -25

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Scientists discuss the role of climate change in the Syrian civil war

  • 02 Mar 2015, 20:00
  • Robert McSweeney

This month will see the civil war in Syria reach an inauspicious fourth birthday. Since the uprising in 2011, over 220,000 people have been killed and almost half the Syrian population have fled their homes.

Evidence suggests the conflict was triggered by a complex mix of social, political, economic and environmental factors. But new research finds that human-caused climate change could also have had an influence.

The study, published in the Proceedings of the National Academy of Sciences, suggests a severe drought that began in 2006 was a catalyst for the conflict, and that climate change has made such droughts in the region more than twice as likely.

Syrian conflict

On 15th March 2011, Syrian security services opened fire on pro-democracy protesters in the southern city of Dara'a, killing several people. The unrest that followed spread throughout the country over the ensuing months, and by February 2012, Syria had descended into civil war.

A study published last year found that a multi-year drought contributed to food shortages, urban migration, and unemployment in the run up to the conflict.

Now the new study says the drought had a catalytic effect on the unrest in Syria, and human-caused climate change has made the chances of such a severe drought between two and three times more likely.

Prof Richard Seager, a climate scientist at Columbia University and co-author on the new study, explains:

"We're not saying drought caused the war. We're saying that added to all the other stressors, it helped kick things over the threshold into open conflict. And a drought of that severity was made much more likely by the ongoing human-driven drying of that region."

Multi-year drought

Syria sits in a band of relatively moist and productive land in the Middle East, known as the Fertile Crescent. But between 2006 and 2010, the region was hit by the worst multiyear drought since 1940.

Syria gets almost all of its rain during its six-month winter, from November to April. In 2007-08, winter rainfall across Syria fell by a third, with some areas receiving no rain at all. The winter was the driest in the observed record, the researchers say.

The decreasing rainfall (shown in the top graph below) combined with rising temperatures (second graph) resulted in a decline in soil moisture (third graph), the researchers say. This had dramatic consequences for Syrian agriculture.

Kelley Et Al (2015) Syria Fig1

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New study directly measures greenhouse effect at Earth’s surface

  • 25 Feb 2015, 18:00
  • Robert McSweeney

Scientists know that greenhouse gases in the atmosphere cause the Earth to warm. But measuring exactly how much heat they trap is harder than you might think.

Previous studies using satellites have established that more heat is entering the atmosphere than leaving it. But a new study goes a step further and directly measures the amount of warming greenhouse gases are producing at Earth's surface.

The paper provides the critical link between rising carbon dioxide concentrations and the extra energy trapped in the climate system, the researchers say.

Greenhouse effect

Joseph Fourier first suggested in the 1820s that gases in the Earth's atmosphere trap heat and help keep the planet warm, coining the term greenhouse effect. Physicist John Tyndall later extended the theory by identifying the gases, such as carbon dioxide and methane, that were responsible for the warming.

Jumping forward a century and a half, we now know a lot more. Using satellites to measure how much of the sun's energy enters the Earth's atmosphere, and how much is reflected or re-emitted back into space, scientists have shown that the difference between the two is increasing. This means the Earth is trapping more heat than it used to, and therefore must be warming.

But while those studies show a widening gap between the energy reaching and leaving Earth, they are unable to directly measure how much warming greenhouse gases are causing at a particular point in time. New research, published today in Nature, shows how scientists have directly been able to measure the warming effect of greenhouse gases at Earth's surface.

Measuring energy

The researchers used a set of instruments to take thousands of measurements at the Earth's surface. The instruments record the longwave energy that is re-emitted by greenhouse gases back towards the Earth's surface, which causes the warming.

Making these sorts of measurements on the ground is difficult, says lead author Dr Daniel Feldman, a geological scientist at the Lawrence Berkeley National Laboratory in the US. With weather systems passing overhead, and temperatures and humidity changing frequently, it's tricky to take energy measurements without other factors getting in the way.

To overcome this problem, the researchers measured temperature and water vapour at the same locations so that their influence on warming could be eliminated from the calculations, leaving just the impact of greenhouse gases.

The scientists used data from 2000 to 2010, collected from two sites in the US: the southern Great Plains and northern Alaska. They chose these sites because of their very different climates, says Feldman. This meant the researchers could investigate both a mid-latitude and a high-latitude location.

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